It has been found that such microphones may be sensitive to interference signals, more in particular high-frequency interference signals. An important source of high-frequency signals, that may interfere with such microphones is a GSM telephone apparatus. It has been found that such apparatus may generate signals having a frequency in the vicinity of 900 MHz and 1.8 GHz, which may give rise to interference signals that are perceptible to the user. The degree of interference may be so serious that the user of a hearing aid cannot make good use of a GSM or DECT telephone apparatus.
It is therefore an important object of the present invention to provide a microphone having an integrated amplifier, in which interfering signals in general, and high-frequency interference signals in particular, as, for instance, caused by GSM telephone apparatuses, are sufficiently suppressed.
To achieve this object, an integrated microphone/amplifier unit according to the invention has capacitively coupled power feed and output connections. Thus, interference signals that may be generated by the microphone are effectively short-circuited and are prevented from being present at the output of the integrated unit. Preferably, this short-circuit is realized to ground. It has been found that a value of about 30 pF already provides a good suppression of more than 20 db for frequencies as they occur during use of a GSM telephone.
A further aspect of the present invention relates to the construction of an amplifier module for such an integrated microphone/amplifier unit in miniature. In a macroscopic embodiment, two capacitive couplings can be rather easily provided by placing two capacitors. However, in miniature embodiments, such as, for instance, those necessary for use in a hearing aid, there is no room for individual capacitors.